Chp 11 DC Generators
Interpoles are connected in series with what five (5) components?
armature field coils field poles load stator
The twisting and bending action of the main magnetic field of a motor or generator, which causes sparking is called _____.
armature reaction
Do shunt field windings have (higher or lower) resistance than series field windings?
higher
How are lap-wound armatures connected?
in parallel
How are wave-wound armatures connected?
in series
How are windings connected in a frogleg-wound armature?
in series-parallel
What does magnetic saturation mean?
that no more magnetic flux can be produced no matter how much field current increases
The type of armature winding and field winding used in DC machines is determined by ______.
the application of the machine
The amount of current flowing through the shunt field windings is limited by ______.
the resistance of the winding
The amount of current flow through the shunt field winding is limited by ______.
the resistance of the winding.
What is residual magnetism used for?
to produce an initial output voltage (inrush current) needed by some generators to start.
Why are brushes made of carbon materials that are softer than the materials used to make the commutator?
to promote wear on the brushes instead of the commutator.
_____ is the pushing and turning force created by the magnetic field of the pole pieces and the magnetic field of the armature.
torque
When it is important that the output voltage remain constant regardless of the load, an electronic _______ is used to adjust the shunt field current by sensing the amount of voltage across the load.
voltage regulator
_____ armatures are used in machines designed for high voltage, low current.
wave-wound armatures
Brush leads are labeled _____ and ______, and are referred to as _______ leads.
A1, A2 armature leads
The _____ rotates with the armature and provides connection between the armature windings and the brushes.
commutator
How is a long shunt generator connected?
connected in series with the series winding, but parallel to the armature.
The commutator is made of _______.
copper segments separated by insulation.
______ is the voltage induced in the armature of a DC machine that opposes the applied voltage and limits the armature current.
counter electromotive force (CEMF)
_____ is the magnetic force developed in an armature of the generator when the generator supplies current to the load making the shaft hard to turn.
counter-torque
______ generators have their series and shunt field windings connected in such a way they aid each other in the production of magnetism.
cumulative-compound-wound generator
Do shunt generators (increase or decrease) their output voltage when a load is added?
decrease
______ generators have their series and shunt field windings connected in such a way they oppose each other in the production of magnetism.
differential compounding-wound generator
Counter-torque is a measurement of ______.
electrical energy produced by the generator
All generators operator on the principle of _____.
electromagnetic induction
_____ support the weight of the armature and keeps the armature centered with the field poles.
end plates
_____ is used to connect the series fields of more than one generator in parallel to another.
equalizer connection
_____ are parts of a DC machine that form the field poles, which provides the magnetic field necessary to operate the machine.
pole pieces
What are the effects of brushes not be aligned using the neutral plane?
power loss damage to the brushes and commutator due to arching and sparking
_____ is the amount of magnetism left in the pole pieces of an object after the magnetizing force has been removed.
residual magnetism
_____ is a variable resistor added in series with the field windings to control amount of current flow through the windings.
rheostat
The individual metal bars that make up the commutator are called _____.
segments
A _____ generator is one in which the field winding is excited by the current (residual magnetism) of the generator itself.
self excited generator
What are two types of generators?
self excited series generator separately excited shunt generator
What does torque depend on?
1) armature current 2) field strength
How can you adjust the speed of a separately excited generator?
1) by increasing the field current 2) by increasing the armature rotation speed
In a DC machine, what are the two basic functions of the commutator.
1) connects the armature windings to the external circuit via the brushes; 2) maintains polarity by ensuring proper direction of current flow through the armature windings.
What are two (2) things can control magnetic polarity of the poles?
1) direction of the field current 2) direction of the wound coils
What are three (3) factors that determine the output voltage of a DC generator?
1) number of turns of wire in the armature 2) strength of the magnetic field created by the poles 3) speed of the rotation
What are two (2) ways to reduce/eliminate sparking?
1) shift the brushes 2) add interpoles
What are the characteristics of a series field winding?
1) they are made with few turns of large wire and have low resistance. 2) they are connected in series with the armature 3) They are labeled as S1, S2
What are the characteristics of a shunt field winding?
1) they are made with many turns of small wire and have high resistance 2) they are connected in parallel with the armature 3) they are labeled F1, F2
One (1) horsepower equals ______ watts.
745.7
The armature windings in a DC machine are connected to the ________.
commutator bars
What do the brushes do?
provide connection to the commutator
Wave-wound armatures never contain more than ______ number of parallel paths regardless of the number of poles it has.
2
When the DC machine is used as a generator, what does the commutator do?
Acts like a rectifier changing the AC voltage produced by the armature into DC voltage before it exits vita the brushes to the external circuits.
_______ are made of copper wires wound around the armature core and connected to a commutator segment.
Armature windings
How are series field windings connected?
Connected in series to the armature and wired in series with the load.
_____ are machines that convert mechanical energy into electrical energy.
DC generator
______ and _____ are terminal leads used for shunt field windings.
F1, F2
What is an advantage of having a separately excited generator?
Magnetic field remains stable
______ and ______ are terminal leads used for series field windings.
S1, S2
What are the two types of DC generators?
Self-excited generator Separately excited generator
Which type of windings has more resistance: series field windings or shunt field windings?
Shunt field windings
When using an ohmmeter, how can you identify the field windings from the armature field windings?
The field windings will have a high resistance reading; the armature windings will have a resistance reading of 10 ohms or less.
How are shunt field windings connected?
They are connected in parallel to the armature and wired in parallel to the load.
T/F: Because the shunt field winding is connected in parallel with the armature, it will have the same voltage as the armature.
True
T/F: Series and shunt field windings are wound around the same pole pieces.
True
How are shunt field windings made?
With many turns of small wire
How are series field windings made?
With relatively few turns of very large wire.
Interpoles used in a generator must have the same polarity as the main field pole directly (ahead or behind) them in the sense of rotation of the armature?
ahead
Type of voltage produced in all rotating armatures is ______
alternating voltage
Series and shunt field windings wrapped around the metal pole pieces are sued to produce ______ in the pole pieces.
an electromagnetic field
In a separately excited generator, an increased output voltage is proportional to _______.
an increase in armature speed
_____ is the rotating member of the DC machine.
armature
What are the main parts of a generator?
armature brushes commutator field poles frame stator
Armature reaction is proportional to _______.
armature current
Counter-torque is proportional to _______.
armature current
Interpoles used in a motor must have the same polarity as the main field pole directly (ahead or behind) them in the sense of rotation of the armature?
behind
_____ ride against the commutator segments and are used to carry the power to the external circuit of a DC machine.
brushes
What are brushes most often made of?
carbon (because it is less resistant to higher temperatures)
_______ is the direct current that flows through the shunt field winding and is used to turn the iron pole pieces into electromagnets.
field excitation current
The armature windings generate voltage by cutting a magnetic field as the armature rotates. This magnetic field is created by using electromagnets called ______ mounted around the periphery of the generator.
field poles
_____ surrounds the working parts of the DC machine providing support and protection.
frame
_____ armatures are most often used in machines requiring moderate voltage and moderate current.
frogleg-wound armatures
What are two (2) disadvantages of a separately excited generator?
inconvenience cost of providing a DC source
Do series generators (increase or decrease) their output voltage when a load is added?
increase
Self excited voltage between terminals decreases faster as the load ______ (increases or decreases).
increases
What does having armature windings placed into core slots do?
increases the magnetic field
______ are small pole pieces connected between the main field poles and are use to help correct armature reaction.
interpoles
What is an armature made of?
laminated iron
_____ armatures are used in machines designed for low voltage, high current.
lap-wound armatures
List the three (3) types of armature windings.
lap-wound armatures wave-wound armatures frogleg-wound armatures
What is the most commonly used type of generator?
long shunt generator
Do series field windings have (higher or lower) resistance than shunt field windings?
lower
Pole pieces are made out of ______, such as ______ and ______.
material of high magnetic permeability such as soft iron and high carbon steel.
Insulation that separates the commutator segments and between shaft and commutator
mica
_____ is the point on the commutator where the armature windings are parallel to the magnetic flux lines of the pole pieces and no induced voltage is produced.
neutral plane
_____ is the point on the commutator where the brushes should be set for proper alignment.
neutral plane
Wave-wound winding armature never have more than ______ number of brushes.
one
What type of generator has its field magnets excited by a current from an external source and must not be exposed to residual magnetism?
separately excited generator
Which is more stable, a self excited generator or a separately excited generator?
separately excited generator
What are the two (2) types of field poles?
series shunt
Compound generators contain _____ and ______ field windings.
series and shunt
What are the two types of field windings.
series field windings shunt field windings
What are two types of compound generators?
short shunt generator long shunt generator
A ______ can be connected in series with the shunt field winding to control the current flow through the winding by adding or removing resistance.
shunt field rheostat
Because one of the factors that determines the output voltage in a DC generator is the strength of the magnetic field, the output voltage can be controlled with a __________ which controls the amount of current flowing through the field.
shunt field rheostat
